CRYSTALLINE STRUCTURE OF SOME SILVER AND COPPER NUGGETS. 257 



the copper has a confused crystalline structure, with a tendency 

 to radiation in parts. The silver has in this case been deposited 

 only upon the surface of copper, in the other instance (Plate *]> 

 fig. 2) it has apparently filled cavities in the copper, which appear 

 to have previously contained other mineral matter, since removed 

 by solution. 



In one specimen which I saw several years ago, and not now 

 accessible to me, (I think that there are similar ones in the collec- 

 tion at the Geological Museum, London) the silver was so scattered 

 through the copper in isolated patches as to present a porphyry tic 

 appearance. This structure used to be explained by assuming 

 that the metals had cooled so slowly in the matrix (melaphyre) 

 that the silver and copper had completely separated and solidified 

 apart from each other. One difficulty felt in this explanation was 

 that no artificial instances of such complete separation of two 

 metals which readily alloy were known. 1 



I think that it is highly probable that such porphyrytic speci- 

 mens, if sliced at right angles instead of in one plane only, would 

 show that the apparently isolated masses of silver have connections 

 with external surfaces of the copper and that it was possible for 

 the silver to have found its way in from solution, as is seen to be 

 the case in Plate 7, fig. 2 ; in this specimen the porphyrytic 

 structure is not so characteristic as those I am referring to, 

 because in the illustration accompanying this paper all the silver 

 masses are seen to reach the exterior either directly or indirectly, 

 hence there is no difficulty in accounting for the relative positions 

 of the two metals. 



Pumpelly 2 in speaking of the deposits of copper as a whole states 

 that the silver appears to have been directly precipitated by the 

 copper, but he does not give any details of the structure, beyond 

 stating that on rolling, the two metals became more or less separated 

 and may be detached from each other. 



1 ? See Am. Journ. Sci. and Art, Vol. in., (1821) p. 201. 



2 Geology of Michigan 1873. 



G— Dec. 5, 1900. 



